In milling cutters having shim plates that are arranged under replaceable milling inserts, the individual shim plate has an important task in preventing—or at least as far as possible counteracting—serious damage in the event of an insert breakdown. Namely, if the co-operating milling insert suddenly would lose its cutting capacity during operation, e.g., as a consequence of fractures or other unexpected faults, the basic body of the milling cutter may dig into the workpiece and cause expensive damage not only to the proper basic body but also to the driving spindle and the parts of the machine tool co-operating with the same. For this reason, the shim plate is mounted in such a way that the edge portion thereof positioned rotationally behind the chip-removing cutting edge of the milling insert, on one hand, protrudes radially a distance in relation to the envelope surface of the basic body, but, on the other hand, is located inside an imaginary circle described by the cutting edge during the rotation of the milling cutter. During normal conditions, i.e., as long as the milling insert is in working order, the peripheral edge portion of the shim plate clears from the generated surface in the workpiece, at the same time as the envelope surface of the basic body positioned rotationally behind the shim plate is situated radially inside an imaginary circle described by the edge portion of the shim plate. Therefore, if an insert breakdown would occur, the peripheral edge portion of the shim plate can proceed to remove chips from the workpiece without the rotating basic body digging into the workpiece. In other words, the edge portion of the shim plate can passably assume the chip-removing function of the milling insert during the short time that is required to interrupt the milling operation before a more extensive tool and machine breakdown occurs. Another task for the shim plate is to form a reliable and accurately located long-term bottom support for the milling insert, at the same time as the receiving seat in the basic body is protected against heat. For this reason, the shim plate is usually manufactured from a material, such as cemented carbide, that is harder than the material of the basic body, which in turn most often is formed of steel or aluminium. The shim plate is connected semi-permanently with the basic body, usually via a tubular screw that includes, on one hand, a male thread that is tightened in a female thread in a hole that mouths in a bottom of a seat of the milling insert, and on the other hand a female thread in which a male thread on the tightening screw can be tightened to fix the milling insert. In contrast to the shim plate—which under good conditions can work during the entire service life of the basic body without needing to be replaced—the milling insert is replaced on repeated occasions. In order to avoid a non-uniform wear of the upperside of the shim plate, the same is face ground at the same time as the underside of the milling insert is allowed to protrude radially a short distance (0.1-0.2 mm) outside the radially outer edge portion of the shim plate. In such a way, it is avoided that the numerous milling inserts gradually coin and deform the upperside of the shim plate.
In many milling cutters, the milling inserts are tipped (and thereby also the shim plates) into particular so-called tipping-in positions axially as well as radially. More precisely, the individual milling insert can be tipped into, on one hand, a positive, axial tipping-in angle, which means that the chip-removing cutting edge is tilted in the direction upward/rearward in the direction of rotation, and on the other hand a negative, radial angle, which allow the clearance surface behind the cutting edge to clear the generated surface in the workpiece. Generally, the milling inserts become more easy-cutting the greater the positive, axial tipping-in angle is.
Problems that above all are associated with such easy-cutting milling cutters that make use of indexable milling inserts mounted in relatively great axial tipping-in angles in the basic body form a basis of one or more embodiments of the invention.
In previously known shim plates having a polygonal basic shape, the uppersides thereof have extended as a continuous, plane surface up to a straight edge or boundary line that forms a direct transition to the connecting side surface. If the angle between the upperside and the side surface amounts to 90° (for milling inserts having a neutral cutting geometry), the edge portion in question of the shim plate obtains a right-angled shape as viewed in cross-section, while the same obtains an acute-angled cross-sectional shape if the angle between the upperside and the side surface is smaller than 90° (for milling inserts having a positive cutting geometry). In both cases, the upperside of the shim plate has to have an area that is somewhat smaller than the area of the underside of the milling insert, because the last-mentioned one—as pointed out above—has to protrude a short distance from the upperside of the shim plate so that the milling insert should not coin and deform the upperside of the shim plate.
When a polygonal, indexable milling insert has a positive cutting geometry, the same may be compared to a truncated and upside-down pyramid, the upperside of which has a larger area (and circumference) than the underside. It is in the same way with the shim plate because the side surfaces thereof should run parallel to or radially inside the clearance surfaces of the milling insert. In other words, the upperside of the shim plate has also a larger area (and circumference) than the underside thereof. For reasons easily realized, the area difference increases between the uppersides and the undersides by increasing nominal clearance angles of the clearance surfaces of the milling insert and the side surfaces of the shim plate, respectively.
A serious shortcoming of previously known shim plates for milling cutters having milling inserts that are tipped in at least medium-sized axial tipping-in angles is that they obtain a mediocre bottom support in the appurtenant seat in the basic body because their supporting undersides become smaller by increasing axial tipping-in angles, and that the edge portions, which serve as auxiliary cutting edges in the event of insert breakdowns, are not very resistant to the abrupt and great stresses that they may be subjected to.
The present invention aims at obviating the above-mentioned shortcomings of previously known shim plates. An object of the invention to provide a shim plate that is suitable for rotatable tools in the form of milling cutters and, on one hand, has an edge portion of great strength and robustness serving as an auxiliary cutting edge, and on the other hand affords a good bottom support in the appurtenant seat. In this connection, the shim plate should be constructed in such a way that the upperside thereof is not coined or in another way deformed in connection with a large number of repetitive exchanges of milling inserts. In addition, the shim plate should afford good precision in respect of its own and the milling insert's three-dimensional position in the basic body. Still another object is to provide a shim plate that performs well also when the co-operating milling insert is tipped in at great axial tipping-in angles.